Bowthorpe EMP Ltd., of Stevenson Road, Brighton, East Sussex, England, have recently marketed with considerable success a surge protector plug comprising a generally standard British style 13 amp 3-pin plug constructed to BS1363 and incorporating a three-element non-linear resistor assembly connected in delta configuration with the three pins of the plug, the arrangement being such that transient overvoltages at any of the plug pins will be substantially instantaneously suppressed by breakdown of a respective one or more of the non-linear resistors so as to conduct the transient to ground. The non-linear resistor assembly of this plug comprises a ceramic tube within which were contained three non-linear resistor discs formed of a zinc oxide based material and each having electrodes provided on their opposed faces, the three zinc oxide discs being stacked within the ceramic tube and metallic conductor discs being interposed between the zinc oxide discs in the stack and provided at the ends of the stack and having lead portions extending out of the stack and out of the ceramic tube and connecting to the respective plug pins. However, whereas no great difficulty was experienced in incorporating such a non-linear resistor assembly into the standard British style plug, difficulties were encountered in similarly adapting the various forms of plugs used as standard in other countries within Europe and also further afield primarily on account of the bulky nature of the non-linear resistor assembly.
To the Applicants knowledge and belief, few previous proposals have been made for the incorporation of non-linear resistor materials into power connectors for the purposes of surge suppression. One such previous proposal is described in U.S. Pat. No. 3,821,686 (Harnden) which discloses several embodiments most of which involve the provision of a two-pin plug or socket connector formed with a varistor block disc or wafer incorporated into the connector body or onto a front surface thereof, and only one of which concerns a three-pin connector with an earth/ground contact. In the disclosed two-pin connectors, the arrangements are either inefficient as regards the electrical connections made between the pins and the non-linear resistor material or are wasteful in terms of the amount of non-linear resistor material utilized; for example, the embodiment wherein the varistor is provided at the front face of the plug has poor provision for reliable electrical contact between the plug pins and the varistor and exposes the varistor to surface contamination with consequent deterioration of its effectiveness. In the disclosed three-pin connector, whilst more economical use is made of varistor material, the varistors being provided in the form of relatively small discs, the connections to the varistors are uncertain and no varistor is provided to accommodate L-N mode transients, and only L-E and N-E modes are provided for.
British Patent Specification GB-A-2 119 182 (ITT Industries Inc.) discloses an electrical connector for signal lines of data processing equipment, and not an electrical power connector. In the arrangement disclosed, a wafer of zinc oxide or other varistor material has a plurality of spaced-apart electrodes on one face and a ground electrode on its opposed face and the plurality of pin contacts provided in the connector each include a spring finger contacting a respective one of the spaced-apart electordes. European Patent Specification EP-A-0018067 (Reliable Electric Company) discloses a line protector for a communications circuit, and again not an electrical power connector. In the arrangement disclosed, a varistor body has an electrode on one face coupled to a ground pin and on its opposite face has a pair of spaced-apart electrodes coupled to respective ones of two line pins, and there are furthermore provided a pair of spring clips which span the thickness of the varistor body and would short the spaced-apart line electrodes to the ground electrode were it not for the provision of an insulating sheet which is adapted to melt under high surge conditions. Such an arrangement would be unsuitable for a power line connector since the occurrence of a transient such as to melt the insulating sheet would place a short-circuit of substantial current carrying capability directly between the live and/or neutral power lines and earth with potentially disastrous consequences.
Other prior art material of marginal interest to the present invention is disclosed in U.S. Pat. No. 3,742,420 (Harnden, Jr.) which discloses a metal oxide varistor wafer with feed-through holes for the electrodes of an electrical device such as a semiconductor MOSFET for protecting the device against transient surges, in U.S. Pat. No. 3,768,058 (Harnden, Jr.) which discloses a metal oxide varistor circuit component comprising a body of defined thickness having a continuous electrode on one surface and a plurality of electrodes on an opposed surface having interelectrode spacings of defined width less than the wafer thickness forming conduction gaps, in U.S. Pat. No. 4,316,171 (Miyabayashi et al) which discloses a titanium dioxide based varistor adapted for use as a noise suppressor in DC motors and comprising an annular body having three electrodes provided on one surface in equally divided sectors and a single annular electrode provided on the opposite surface, and in U.S. Pat. No. 4,212,045 (Martzloff) which discloses a multiterminal varistor configuration particularly adapted for the protection of polyphase electrical circuits such as low-voltage polyphase AC motor. None of the aforesaid patents concerns the provision of transient protection in mains power supply connectors.